The action by a three-judge panel of the U.S. Court of Appeals for the 9th Circuit overruled a federal judge's go-ahead for the issuance of same-sex wedding licenses. Earlier this month, U.S. District Judge Vaughn R. Walker invalidated Proposition 8, the state's voter-approved ban on such unions, finding that the measure was discriminatory and violated the Constitution.

Supporters of the law appealed Walker's finding to the 9th Circuit, and the appeals court ruled Monday that Proposition 8 could remain in effect while it considers the case. It indicated that it will act relatively swiftly on the appeal, setting a hearing for early December -- a schedule that pleased those challenging the measure. But it will not come soon enough for the gay and lesbian couples who were already making plans to exchange vows at city halls around the state this week.

Groups that defended Proposition 8 in court applauded Monday's order.

"Invalidating the people's vote based on just one judge's opinion would not have been appropriate, and would have shaken the people's confidence in our elections and the right to vote itself," said Andy Pugno, a lawyer for ProtectMarriage.com.

Marriage Equality USA, a pro-gay-marriage group, had helped arrange for champagne, music, cakes and officiators to be available at San Francisco City Hall by 5 p.m. Wednesday, when Walker ordered that the marriages could resume.

"Oh, my goodness, this movement is like a labyrinth. Just when you think you've reached the center, the design just shoots you out to another path," said Molly McKay, a spokeswoman for the group. "But you know you're circling back to where you want to go, which is to end discrimination and bring in marriage equality."

Supporters of Proposition 8 say that the ballot initiative, which passed with 52 percent of the vote two years ago, is valid because it reflects the will of the people. The groups also say that exclusively allowing heterosexual marriage is in the interest of the state because of its role in procreation.

But Walker agreed with gay rights groups that Proposition 8 discriminated against same-sex couples and violated the Constitution's 14th Amendment.

He also questioned whether the nonprofit and religious groups defending the initiative in court have standing to appeal his decision, because they are not named as defendants. California Gov. Arnold Schwarzenegger (R) and Attorney General Edmund G. "Jerry" Brown Jr. (D), both of whom oppose Proposition 8, declined to defend the initiative in court.

In its one-page ruling Monday, the appeals court suggested that it, too, has questions and asked the anti-gay-marriage groups for evidence that they have a right to appeal the decision.

Groups on both sides of the issue called Walker's ruling on the constitutionality of Proposition 8 one of the most significant rulings in the battle over same-sex unions. The case is widely expected to end up before the U.S. Supreme Court.

Ballot initiatives have been a valuable tool for opponents of same-sex marriage, because every time the issue is put before voters, they agree to define marriage as between a man and a woman.

Gay rights groups have been more successful in the courts. Earlier this summer, a federal judge in Massachusetts invalidated the ban on federal recognition of same-sex unions in two cases that are also expected to be appealed.

Five states and the District allow same-sex couples to legally marry, while a majority of states explicitly ban gay unions.